A wireless sensor network (WSN) consists of spatially distributed autonomous sensors that cooperatively monitor physical or environmental conditions, such as temperature, sound, vibration, pressure, motion or pollutants. WSNs are used in many industrial and civilian application areas, including industrial process monitoring and control, machine health monitoring, environment and habitat monitoring, healthcare applications, home automation, and traffic control. Each wireless sensor node in a WSN is typically equipped with sensors, a radio transceiver or other wireless communications device, a small microcontroller, and an energy source, usually a battery. For a WSN application, the use of an ultra low power wireless sensor node is an important way to gather and manipulate information from the sensor for a sufficiently long period with a limited battery energy source after initial installation. Among the components inside a wireless sensor node, the wireless radio transceiver generally consumes most of the power since its operation frequency is in the range from hundreds MHz to several GHz.
IEEE 802.15.4 is the standard that is used for the WSN application. IEEE 802.15.4 (Low Rate WPAN) and its variant Zigbee, an industry standard, deals with low data rate but very long battery life (months or even years) and very low complexity. The first edition of the 802.15.4 standard was released in May 2003. The ZigBee set of high level communication protocols is based upon the specification produced by the IEEE 802.15.4 taskgroup. Minimum Shift Keying (MSK), a kind of Offset Quadrature Phase Shift Keying (OQPSK) is used as the modulation scheme of IEEE 802.15.4. For the generation of MSK modulated signal, a set of complex reference signals are required. Thus, for the demodulation of the MSK modulated signal, a complex reference signal is also needed.
FIGS. 1A and 1B show the conventional heterodyne and homodyne radio receiver architecture respectively for complex modulated signal detection. The heterodyne radio transceiver architecture has been popular for the implementation of radio transceiver with many discrete and partially integrated circuit components. As shown in FIG. 1A, the heterodyne radio receiver architecture 10 consists of multiple components including a radio frequency filter 11, an input matching network 12, two complex mixers 14 and 16, two complex variable gain amplifier (VGA)/low-pass filter (LPF) components 15 and 17, a low noise amplifier (LNA) 13, and a complex analog-to-digital (A/D) converter 18, which outputs a signal in I/Q (in-phase and quadrature-phase) format. The heterodyne architecture also consists of two local oscillator signal (LO) buffers 19A and 19B. However, this complicated heterodyne radio architecture is not adequate for an ultra low power radio application like WSN.
On the other hand, the homodyne radio architecture 20 in FIG. 1B is more simplified in its architecture than the heterodyne radio architecture and consists of a radio frequency filter 21, an input matching network 22 and a LNA 23. It also requires a complex mixer 24, a complex VGA/LPF 25 and a complex A/D converter 28 as the power consuming function blocks. The A/D converter 28 outputs the signal in I/Q format. The power consumption of a homodyne receiver is reduced significantly compared to the heterodyne architecture and is also adequate for the implementation of the intensively integrated radio receiver.
However, the homodyne architecture in FIG. 1B has its limitations for the implementation of an ultra low power radio receiver. As shown in FIG. 1B, a LNA 23, a complex mixer 24 and its complex local oscillator signal (LO) buffer 29, which operate at a very high frequency and consume the most power, are the main architectural defects of the homodyne radio architecture when used for an ultra low power application. Recently, there were some research reports that removed the power consuming high frequency LNA with marginal amounts of noise performance degradation. Even with this mixer-first homodyne receiver architecture, having a complex mixer and its complex LO signal buffers, a complex VGA/LNA, and a complex A/D converter limit the power efficiency of a homodyne architecture-based receiver, because they are required for the detection of a complex signal. Essentially, a complex signal path receiver consumes almost two times more power than a single path receiver since a complex signal path receiver has two single path receiver chains for the complex I/Q (in-phase and quadrature-phase) signal detection. However, a single path receiver cannot be used for the radio systems using a complex modulation like a MSK (minimum shift keying) modulation because it causes loss in detection.
In summary, there is a need for an IEEE 802.15.4 compliant wireless receiver which has low power consumption and at the same time has good noise and signal-to-noise performance.